This field lists the message ID's of any articles prompting the submission of this article. It is required for all follow-up articles, and forbidden when a new subject is raised. Implementations should provide a follow-up command, which allows a user to post a follow-up article. This command should generate a Subject line which is the same as the original article, except that if the original subject does not begin with "Re: " or "re: ", the four characters "Re: " are inserted before the subject. If there is no References line on the original header, the References line should contain the message ID of the original article (including the angle brackets). If the original article does have a References line, the followup article should have a References line containing the text of the original References line, a blank, and the message ID of the original article.
The purpose of the References header is to allow articles to be grouped into conversations by the user interface program. This allows conversations within a newsgroup to be kept together, and potentially users might shut off entire conversations without unsubscribing to a newsgroup. User interfaces may not make use of this header, but all automatically generated followups should generate the References line for the benefit of systems that do use it, and manually generated followups (e.g. typed in well after the original article has been printed by the machine) should be encouraged to include them as well.